Heretofore, the prior art has included seat or seal assemblies for butterfly valves in which both metal and elastomeric sealing surfaces have been provided. For example, U.S. Pat. No. 3,986,699 dated Oct. 19, 1976 and an article on pages 95 and 96 of "Chemical Engineering" dated Oct. 10, 1977 show butterfly valves in which a seat assembly includes both metal and elastomeric sealing surfaces for contacting the valve disc in the closed position thereof. However, both of the prior art structures show arrangements in which fluid pressure from the upstream side is provided behind the seal assembly to aid in urging the seal assembly inwardly into engagement with the adjacent sealing surface of the disc. In addition, a separate elastomeric backing ring is provided in these prior art structures behind the seal assembly to urge the seal assembly inwardly into tight engagement with the adjacent peripheral sealing surface of the valve disc.
It is highly desirable not to have to rely on fluid pressure getting behind the seal assembly in order to urge the seal assembly inwardly as there is a possibility that the seal assembly could malfunction and perhaps fluid would be obstructed in getting behind the seal assembly. Even in the event of a separate O-ring being provided behind the seal assembly, such an O-ring might be damaged or at least partially consumed during a fire or the like and could not be relied upon to urge the seal assembly inwardly as might be necessary under emergency conditions. Also, the fluid pressure on the upstream or downstream side might not be sufficiently high to pass between the sides of the groove and the valve assembly for flowing behind the seal assembly.